1. Field of the Invention
This invention relates to a triaxial testing apparatus, more particularly to an improved base of a specimen cell of a triaxial testing apparatus.
2. Description of the Related Art
FIG. 1 illustrates a conventional triaxial compression or extension testing apparatus for studying the properties of unsaturated soil, gravel and pebble soil, or soft rock. The triaxial compression test is carried out in cooperation with a universal testing machine (not shown), and a triaxial cell 12 which is mounted on the universal testing machine and which accommodates an unsaturated soil specimen that has a size smaller than or equal to 2.5″. The triaxial cell 12 has a cylindrical cell body 121 defining a pressure chamber 122, a top seat 123 closing a top end of the pressure chamber 122 and connected to the universal testing machine, and a bottom seat 124 closing a bottom end of the pressure chamber 122. The bottom seat 124 has a porous ceramic plate 125 disposed within the pressure chamber 122. A specimen cell 11 is disposed within the pressure chamber 122, and has a peripheral wall 113 with a bottom end connected to the bottom seat 124, and a top platen 111 disposed on top of the peripheral wall 113. The soil specimen is disposed in the specimen cell 11, and is supported by the porous ceramic plate 125. Pressurized water 13 is disposed within the pressure chamber 122 between the specimen cell 11 and the cell body 121.
The universal testing machine uses a piston 14 to apply an external compression or extension force to the top platen 111 so as to provide a deviator stress σd. The pressurized water 13 provides a confining pressure σc to the specimen cell 11. During an experiment, a stressor (not shown) provides a pore air pressure ua through the top platen 111 and a pore water pressure uw through the porous ceramic plate 125. Since the pore air pressure ua is greater than the pore water pressure uw, it forces moisture in the soil specimen to flow down to the porous ceramic plate 125.
Although the study of the properties of the unsaturated soil can be achieved using the conventional triaxial compression testing apparatus, the following drawbacks arise:
1. When the diameter of the soil specimen is greater than 2.5″ and the compression strength of the soil is high, the deviator stress σd provided by the universal testing machine is mostly and directly transmitted to the porous ceramic plate 125. Further, when the soil specimen contains granular materials of different mass and size, such as gravel and pebble soil or rock, stress concentration can occur in the porous ceramic plate 125, so that the porous ceramic plate 125 is easily ruptured which leads to test failure.
2. When the pressure difference between the pore air pressure ua and the pore water pressure uw is excessively large, the porous ceramic plate 125 is also easily ruptured.